As is well known, the use of adhesives is important in the construction of footwear. Various adhesives are used in affixing the layers and components of the shoe together. For instance adhesives are used in (i) bonding layers of the upper together, (ii) holding the upper to the insole, (iii) maintaining the shape of the shoe once on the last by bonding folds of the upper to itself, and (iv) bonding the upper to the sole.
Adhesives used for shoe assembly are generally employed in one of three ways in the footwear industry. The term "shoe" is used throughout to refer to any product intended to be worn on the foot and produced by the footwear industry. As such it should not be read to be particularly limiting and is intended to include footwear such as boots, socks, and slippers. Adhesives are frequently employed in the so-called cold bonding process, the heat reactivation process, and by application of the adhesive in the melt.
Cold bonding indicates the process where a solvated adhesive is applied to one or both of the substrates, or adherends, to be bonded. Without heating, the layer of adhesive is kept uncovered for a sufficient length of time to allow most of the solvent contained in the adhesive to evaporate. The layers to be bonded are brought into contact while there is sufficient residual solvent contained in the adhesive to provide an effective bond.
Heat reactivation is the process whereby the adhesive is applied to the desired adherend(s) by whatever means most efficient, heating to the required temperature and then immediately combined. The method of heating is generally infrared radiation or hot air. The temperature required to heat-reactivate the adhesive is noticeably not the transition to take the adhesive totally into the melt but is rather a lower temperature. Adhesives known in the art for heat reactivation are generally hydroxyl polyurethanes made from polyesters which have a strong tendency to crystallize. The crystalline segment of the polyesters is melted but not the entire polyurethane to provide the heat reactivation characteristics of the adhesive. For a discussion of this prior art technology see for instance "Polyurethane Handbook", by Gunter Oertel, 1985, Carl Hanser Verlag and U.S. Pat. No. 3,718,518.
A melt application of the adhesive can be achieved in numerous ways; for instance, application of the adhesive by solvent and subsequent heating of the residual solids, and combining of the adherends while the adhesive is in a molten phase. Optionally, this might include direct application of the adhesive as a hot melt.
As is well known to those in the footwear industry, not to mention countless users of their products, comfort of the shoe is desired by the wearer. Numerous variables affect the wearer's comfort. Such issues considered are the fit of the shoe, aesthetic appearance, shock absorbency and the microclimate around the foot. See for instance the summary regarding foot comfort by Bunten, J.,"Foot Comfort", 1983, SATRA report SR 44.
The inventive concepts herein concern themselves with the microclimate surrounding the foot and the associated comfort or lack thereof to the wearer. For simplicity herein comfort will therefore concern itself with this microclimate. Comfort is established by the removal of moisture from the area surrounding the foot. As has been well documented in the past this is accomplished by the shoe absorbing and transmitting moisture.
Frequently a compromise in comfort must be balanced by the shoe manufacturer in the assembly of the shoe. For instance comfort may be traded for other factors such as design features, material selection, manufacturing techniques, and cost, to name a few.
In the past, in efforts to provide waterproof footwear, breathable waterproof laminates of, for example, expanded poly(tetrafluoroethylene), sometimes hereinafter ePTFE, have been used in various cofigurations. See, for instance U.S. Pat. No. 4,599,810. The inherent comfort features provided for by these breathable waterproof laminates have frequently been compromised through the use of non-breathable adhesives smeared over the breathable surface area of the laminate thus precluding moisture vapor passage. This renders the final product uncomfortable. The lack of the ability of conventional, non-breathable adhesives to transfer moisture away from the foot has caused discomfort to the wearer by creating a moist, hot microclimate around the foot.
The prior art has attempted to address this situation in a number of ways
Elimination of the adhesive altogether, at least in the assembly of the upper, has been tried. This can be accomplished by using the laminate as the only material in the upper. Obvious limitations to options concerning final shoe usage, aesthetic appeal, and fashion have resulted from pursuing this path.
Utilizing alternative fastening techniques available in the trade, specifically stitches and tacks, create another problem. These fastening techniques defeat the waterproof characteristic of the laminate through the generation of holes. These holes in turn must be eliminated to restore the waterproofness.
Minimization of the area of the breathable, waterproof laminate that is covered by the adhesive has been tried. This is done usually when the laminate is the principal component of the shoe as stated above, or when it is sandwiched between shape-holding layers. Where adhesive is still required its damaging influence can be minimized by using it in a discontinuous manner. The adhesive, for example can be applied during assembly only at the borders of the adherends. Alternatively if a greater area is required to be bonded, the adhesive can be applied discontinuously over this area by employing a pattern such as a dot or line pattern currently practiced. In addition adhesive webs have been employed to achieve discontinuity in the adhesive layer. U.S. Pat. No. 3,713,938 taught that the correct combination of the amount of solvated adhesive, substrate, and combining technique achieved a discontinuous adhesive layer to prevent excessive loss of water permeability.
U.S. Pat. No. 3,398,042, to Odenthal, et. al, teaches the use of a breathable adhesive in assembly of shoes of leather to avoid the loss of the inherent breathability of the starting leather materials. Hydrophilic hydroxyl terminated polyurethane materials having an ethylene oxide content of at least 85% as disclosed in U.S. Pat. No. 3,398,042, are inherently weak materials, particularly when wet, and require crosslinking. This patent makes it clear that a second component is necessary in addition to their material to make a final product. This creates for the shoe manufacturer concomitant additional steps and a time delay on the order of days before the adhesive provides its final properties. This time delay further illustrates that the properties are heavily dependent on the crosslinking of their material to provide the necessary adhesion characteristics. Furthermore, achieving the sufficient strength through crosslinking, lowers significantly the moisture transmitting characteristics of these materials. As such, the shoe manufacturer has a more difficult adhesive to fit into manufacturing and, in turn, does not realize all the benefits that conceptually a breathable adhesive offers. A "one-component"adhesive that does not need to be chemically crosslinked would be desirable.
Most of the approaches reviewed supra have been of import in offering comfortable footwear to the marketplace. More specifically these approaches have been employed to various degrees for waterproof, breathable footwear. Unfortunately all of these practices have concomitant limitations. The manufacturer of comfortable footwear is limited by one or all of the following limitations: (1) the need to modify in undesirable ways his manufacturing techniques and/or equipment, (2) reducing the design options possible both in style and in material selection, and (3) compromising the comfort in use to the wearer of the final footwear product.
Obviously, these limitations are less than desirable in the pursuit of the optimum comfort in functional footwear. By way of example, men's dress shoes are expected to be made of fine leather. In creating a functional waterproof shoe through the use of breathable, waterproof laminates, such as ePTFE laminates, an adhesive is used to assemble the laminate liner to the leather upper. The adhesive, however, being non-breathable, compromises all that has gone into creating comfortable leathers and ePTFE laminate combinations. This is undesirable.
The present invention provides a more satisfactory option to the shoe manufacture in assembly of comfortable shoes than the art heretofore available.